<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>textarea自适应高度</title>
</head>
<body>
<script src="jquery-1.12.2.min.js"></script>

<textarea name="text" class="cDiv" style="width: 300px;"></textarea>
<textarea name="text" style="width: 300px;">textarea是一个文本框，谷歌、IE、Opera默认显示两行文本，火狐默认显示3行文本，行数超过两行是会出现滚动条，可以自己设定设定一些行数来进行强制多少行进行显示。textarea是一个文本框，谷歌、IE、Opera默认显示两行文本，</textarea>
<script>


  $(function () {
    //选取元素 为每一个textarea绑定事件使其高度自适应
    $.each($(".cDiv"), function(i, n){
      autoTextarea($(n)[0]);
    });
  });

  /**
   * 文本框根据输入内容自适应高度
   * {HTMLElement}   想要改变的输入框元素对象
   * {Number}        设置光标与输入框底部保持的距离(默认0)
   * {Number}        设置最大高度(可选)
   */
  var autoTextarea = function (elem, extra, maxHeight) {
    extra = extra || 0;
    var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
      isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
      addEvent = function (type, callback) {
        elem.addEventListener ?
          elem.addEventListener(type, callback, false) :
          elem.attachEvent('on' + type, callback);
      },
      getStyle = elem.currentStyle ?
        function (name) {
          var val = elem.currentStyle[name];
          if (name === 'height' && val.search(/px/i) !== 1) {
            var rect = elem.getBoundingClientRect();
            return rect.bottom - rect.top -
              parseFloat(getStyle('paddingTop')) -
              parseFloat(getStyle('paddingBottom')) + 'px';
          };
          return val;
        } : function (name) {
          return getComputedStyle(elem, null)[name];
        },
      minHeight = parseFloat(getStyle('height'));
    elem.style.resize = 'both';//如果不希望使用者可以自由的伸展textarea的高宽可以设置其他值

    var change = function () {
      var scrollTop, height,
        padding = 0,
        style = elem.style;

      if (elem._length === elem.value.length) return;
      elem._length = elem.value.length;

      if (!isFirefox && !isOpera) {
        padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));
      };
      scrollTop = document.body.scrollTop || document.documentElement.scrollTop;

      elem.style.height = minHeight + 'px';
      if (elem.scrollHeight > minHeight) {
        if (maxHeight && elem.scrollHeight > maxHeight) {
          height = maxHeight - padding;
          style.overflowY = 'auto';
        } else {
          height = elem.scrollHeight - padding;
          style.overflowY = 'hidden';
        };
        style.height = height + extra + 'px';
        scrollTop += parseInt(style.height) - elem.currHeight;
        document.body.scrollTop = scrollTop;
        document.documentElement.scrollTop = scrollTop;
        elem.currHeight = parseInt(style.height);
      };
    };

    addEvent('propertychange', change);
    addEvent('input', change);
    addEvent('focus', change);
    change();
  };
</script>
</body>
</html>